The head of the Attorney-General's Department has rejected claims that funding has been taken from the Royal Commission into Institutional Responses to Child Sexual Abuse to give to the home insulation royal commission.

The child sexual abuse royal commission is operating on a budget of $377 million until mid-2016.

Prime Minister Tony Abbott has described it as "the best-funded royal commission in Australia's history".

Documents provided to the Senate have shown that late last year, $6.7 million was redirected from the child abuse inquiry and put towards the home insulation royal commission.

"We need to know that this Government is standing fully behind the royal commission," he said.

In Senate Estimates Committee hearings, the secretary of the Attorney-General's Department, Roger Wilkins, has provided an explanation.

"There should be no suggestion that funding has been taken away from the child abuse inquiry that it needed or without its knowledge," Mr Wilkins said.

"Savings ($4 million) did arise as a result of fit-out works coming in under budget," he added.

A further amount of $2.7 million "for legal costs and related expenses for witnesses" was also re-used, Mr Wilkins said.

A departmental official has told the Senate the $2.7 million had been set aside for the Commonwealth's own representation at the royal commission but was not required.

Mr Wilkins told the Senate he was responsible for managing the department's budget and that any underspent money either needed to be reallocated to other purposes or returned to the Government's consolidated revenue.

"Where underspends arise, those can be applied to other areas," he said.

Attorney-General George Brandis has dismissed Labor's querying of the underspend.

"There were some very misleading and false remarks by the shadow attorney-general this morning. We are correcting false statements (by Mr Dreyfus)," he said.

The chief executive of the Royal Commission into Institutional Responses to Child Sexual Abuse, Janette Dines, has told the committee that 186 witnesses have so far appeared, but most have been from institutions, not individual victims or "survivors".

She says this has contributed to a lower rate of claims for legal assistance than originally anticipated.

Labor senator Kim Carr, who has led questioning at Senate estimates, has accused Senator Brandis of "clearly misleading" the Senate during its February hearings.

In the February hearings, Senator Brandis had indicated that "no money has been taken away" from the sexual abuse royal commission, although he did add that "there was an underspend that was reallocated".

That evidence has been backed-up by Mr Wilkins and other departmental officials today.